This invention relates to improved apparatus for the examination and evaluation of the process of electrodeposition. More particularly, this invention relates to apparatus for determining the internal stress within electrodeposited metals.
The electrodeposition of metal is utilized both in the field of electroplating wherein a layer of metal is electrolytically deposited on the surface of a metal part and in the field of electroforming wherein a part is formed by electrolytically depositing metal on a mandrel or base and then separating the deposited metal from the mandrel. In either case, it is well known that stress is created within the deposited metal, with such stress being either compressive or tensile. The type of stress encountered (compressive or tensile) and the magnitude of the stress is a function of the composition of the solution used in the electrodeposition process (e.g., the major constituents of the electrolytic plating solution, additives such as wetting agents, and metallic and organic impurities finding their way into the plating solution), and is further dependent on the solution temperature, the current density utilized during the electrodeposition process, and the agitation used while the electrodeposition is taking place. In electrodepositing nickel for example, commercially electroformed parts typically have an internal stress of 2,000-5,000 psi (approximately 14 to 35 meganewtons/meter.sup.2) with stress exceeding 10,000 psi (69 MN/m.sup.2) being encountered if the electroforming process is not carefully controlled.
With respect to electroplating, such internal or "residual" stress can cause appearance defects such as small cracks and blemishes and can cause the electroplate to become less corrosion resistant. With respect to electroformed parts, residual stress is of even greater importance since it can cause structural failure of the electroformed part. In particular, electroforming is often used to produce parts having precise dimensional tolerances with such parts often having regions of relatively thin cross sectional area. If the plating process causes high residual stress in such parts, the parts may crack or may deform such that the parts do not comply with the required dimensional tolerances. Further, in situations wherein the electroformed parts are intended for use at an elevated temperature, or intended for cyclic operation over a substantial temperature range, such deformation or cracking may not be evidenced until the electroformed parts have been placed in service.
Because of the serious effects that can result from residual stress, the measurement of residual stress is important in both the process of electroplating and electroforming. In particular, measuring residual stress is important in establishing basic electrodeposition parameters such as the current density to be employed in a production electrodeposition process and is further important in periodically determining the condition of the electrodeposition solution when such solution is repetitively used. Such periodic testing of the electrodeposition solution, or electrolyte, is necessary since the production of electroplated or electrodeposited parts is generally a "batch" process wherein a large number of parts are simultaneously electrodeposited in a substantial volume of electrolytic solution, and the residual stress of the electrodeposits formed within such a solution continually increases as the solution is repeatedly used. The increase in residual stress from one bath of electrodeposited parts to the next is primarily caused by an increase in the level of contaminants within the electrolytic solution, with such increase in contaminant concentration being caused by a number of factors such as impurities released from the metal anodes as the anodes are depleted to supply metal ions to the electrolytic solution, and impurities that are introduced into the electrolytic solution by pressurized air that is commonly injected into the electrolytic solution to provide agitation.
Due to the relatively large volume of electrolytic solution utilized in a production electrodeposition process it is both wasteful and costly to prematurely replace or replenish the electrolytic solution. Further, since the electrodeposition process must be conducted with the electrolytic solution at a relatively constant elevated temperature, such replenishment or replacement causes interruptions in the production process. On the other hand, since a large number of parts are simultaneously electrodeposited in each production operation, and such production operation requires the expenditure of a substantial amount of time and material, it is both wasteful and costly to continue the production operation until the residual stress within at least one batch of the produced parts exceeds the desired limits. Accordingly, it can be recognized that apparatus for rapidly determining the residual stress that will result during the next ensuing production process is highly desirable. With such an apparatus, the condition of the electrolytic solution can be periodically monitored with little interruption of the production process and the electrolytic solution replaced or replenished only when necessary.
The most widely used prior art instrument for determining the residual stress of electrodeposits is an apparatus commonly called the Brenner-Senderoff contractometer which is described in U.S. Pat. No. 2,568,713 issued to Abner Brenner. In the Brenner-Senderoff contractometer, a cylindrical substrate comprising a spiral-wound strip of metal, sometime called a helix, extends downwardly into the plating solution to be tested with the spiral-wound substrate or helix electrically connected to serve as a cathode of a galvanic plating cell. The upper end of the helix is securely clamped to a support plate, and the lower region of the helix is rigidly clamped to a circular plug. A rod extends upwardly through the interior region of the helix with the lower end of the rod connected to the center of the circular plug and the upper end of the rod extends through the support plate. As metal is electrodeposited on the outer surface of the helix, the stress within the electrodeposit causes changes in the radius of curvature of the helix, i.e., the helix attempts to wind or unwind, and hence causes the rod to rotate. In particular, the helix unwinds under the influence of tensile stress within the electrodeposit to rotate the rod in one direction and the helix winds up under the influence of compressive stress to rotate the rod in the opposite direction.
To provide an indication of the magnitude of the stress, the upper terminus of the rod is affixed to a segmental gear which meshes with a pinion that is equipped with a pointer. As the rod rotates, the gear system provides a tenfold amplification of the angular displacement of the rod and the pointer moves across a calibrated disc that is mounted beneath the pointer. A particular angular deflection, as indicated by the pointer and the graduations of the calibrated disc, can be converted to a stress measurement by a series of mathematical calculations involving the characteristics of the particular helix employed, the type of metal being electrodeposited, and the amount of metal deposited during the test.
Although the Brenner-Senderoff contractometer provides generally satisfactory results, certain disadvantages are associated with its use, especially its use in a production environment. First, the determination of residual stress with the Brenner-Senderoff contractometer is a rather time-consuming process, often requiring 4 or 5 hours for the necessary calibration of the device, electrodepositing metal on the contractometer helix, determining the amount of metal deposited, and performing approximately 20 calculations to convert the resulting dial reading to the stress within the electrodeposit. Such a lengthy testing procedure is not advantageous in the efficient electroplating or electroforming of production parts. Further, since the stress within the electrodeposit is a rather strong function of temperature, it is often difficult to ensure that the temperature of the electrolytic solution of interest remains constant through both the testing procedure and through any ensuing production of parts.
Secondly, the mathematical calculations necessary during the calibration of the contractometer and the determination of the stress from the dial indication are often not easily understood or accomplished by personnel trained in production electroplating or electroforming operations. Largely because of the complexity of the mathematical calculations and the time required to conduct a stress measurement, the Brenner-Senderoff contractometer has been considered by many to be a laboratory instrument and not suited for use in a production plating environment.
Thirdly, use of the prior art contractometer is somewhat limited in that certain measurement inaccuracies occur and it is often difficult to obtain the same stress measurement when two samples of the same electrolytic solution are tested under seemingly identical conditions. These inaccuracies occur largely because of friction within the contractometer gear mechanism and dial pointer. Since the torque caused by a particular electrodeposited sample is often on the order of 10 grams-centimeters, rather small frictional forces or binding in the dial mechanism will cause substantial error. Even though it has become accepted practice to tap the contractometer to eliminate possible binding, and to utilize jeweled bearings without a lubricant to minimize friction, such errors are often encountered. In particular, although the prior art contractometer is generally recalibrated with each use, changes in frictional forces within the dial assembly have been found to cause variations in test results that exceed 10% when the same electrolytic solution is tested under identical test conditions.
Accordingly, it is an object of this invention to provide apparatus for rapidly determining the residual stress created within electrodeposits formed within a particular electrolytic solution.
It is another object of this invention to provide apparatus for determining stress within an electrodeposit that is amenable to use within either a test laboratory or a production shop.
It is yet another object of this invention to provide a spiral contractometer and associated apparatus for determining residual stress within electrodeposited metal without requiring a determination of the amount of electrodeposit or lengthy mathematical calculations.
It is still another object of this invention to provide a spiral contractometer for measuring residual stress within electrodeposited metals wherein the contractometer does not require a gear driven dial indicator.